1. Field of the Invention
This invention relates to novel methods and means for obtaining polymeric bodies having selected morphological properties and the polymeric bodies so produced. More particularly, this invention relates to a process in which a glassy state is induced in a polymer that normally displays properties of plastic flow, without fundamental dependence on the temperature of the polymer. This permits working the polymer while it is in a glassy state to facilitate obtaining polymeric bodies having the desired morphological properties, and then, if desired, restoring the polymer to its normal state of plastic flow.
2. Description of the Prior Art
The present invention is primarily based on the discovery that certain morphological properties can be developed in polymeric materials if the polymeric materials are formed or otherwise manipulated while they are in a glassy state. (As used herein, the terms "polymers" and "polymeric materials" refer to polymers that are derived from either natural or synthetic materials.) In order to induce a glassy state, it is necessary to alter the rheological properties of the polymer, such as elastic deformation and flow.
Without resorting to a chemical modification of a polymeric chain, there are basically several methods available that may be used to alter the rheological properties of a polymer. These methods may make use of the inclusion of solid filler materials, plasticizers and antiplasticizers, or may be dependent upon adjusting the temperature of the polymer.
By way of example, it is generally believed that when finely divided fillers are dispersed amongst the molecular groups of a polymer, the fillers impede the organization of the molecular groups into what would otherwise be their natural configuration. Depending upon the characteristics and the amount of the filler material used, numerous changes in the rheological properties of the polymer can be made by this physical interposition of the fillers.
Plasticizers, on the other hand, may be viewed as internal lubricants that provide the molecular configuration with a greater freedom of movement. Plasticizers generally will have the effect of softening the polymer, improving crystallization resistance, increasing low temperature flex, and reducing the brittle temperature. Antiplasticizers increase tensile modulus and tensile strength with an attendant reduction in ultimate elongation and impact strength.
The effect of temperature upon the polymer, unlike the addition of the foregoing, is a reversible function and may cause variations of the properties of the polymer in either direction. Quite generally, as the temperature of a polymer is reduced, it stiffens, progressively moves to a glassy state and becomes more friable. When the temperature of most polymers is increased, they soften and, if thermoplastic, will approach or enter a state of viscous flow. Thus, for example, by making use of this phenomenon at its two extremes, cryogenic temperatures can be utilized to grind otherwise intractable polymers and temperatures above the melting point can be utilized to shape articles such as by extrusion or molding.
It has now been found that a glassy state can be induced in polymers that normally exist in the plastic state without reliance upon reducing the temperature of the polymer to its glass transition temperature and without permanently altering the properties of plastic deformation and flow. Thus, in essence, the present invention proposes a novel reversible process for altering the apparent glass transition temperature of a polymer, which, in turn, makes it possible to obtain polymers having certain desired morphological properties that either are not known to the prior art or which cannot conveniently be obtained by prior art processes.